#define _CRT_SECURE_NO_WARNINGS 1

//https://leetcode.cn/problems/UHnkqh/description/

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(head == nullptr || head->next == nullptr)return head;
        ListNode* fast = head->next->next,* slow = head->next;
        head->next = nullptr;
        while(slow)
        {
            slow->next = head;
            head = slow ;
            slow = fast;
            if(fast)
            fast = fast->next;
        }
        return head;
    }
};